Green’s Dictionary of Slang

1. to go away, to retreat, to stop talking, esp. as imper.

1901		‘Hugh McHugh’ John Henry 38: When a fellow [...] runs into a lot of low-foreheads he has to back up.
1912		Ade Knocking the Neighbors 131: ‘Back up, Angie!’ she exclaimed.
1919		C.H. Darling Jargon Book 3: Back up—Do not tell any more of that stuff to me.
1935		C.G. Finney Circus of Dr Lao 53: Back up a little, everybody.
1946		Mezzrow & Wolfe Really the Blues 217: Fifth cat: Back up boy, forty-five feet.
1955		B. Schulberg On the Waterfront (1964) 55: Back up, Mac, I like the kid.
1976		P. Theroux Family Arsenal 111: ‘Back up,’ said Hood striding over to the girl.
1981		H. Gould Fort Apache, The Bronx 138: Back up Jim, you’re talkin’ about the woman I love.

2. (US) to back down.

1913–14		Van Loan ‘Easy Picking’ in Taking the Count 301: ‘Backing up, are you?’ sneeered Old Bird.
2000		W.D. Myers ‘A Story in Three Parts’ in 145th Street 121: Now, they said he wasn’t a particularly cruel man [but] he had done what he had done [i.e. sold his slaves’ children] and he wasn’t backing up .

3. (drugs) to refuse to sell drugs on the premise that the purchaser might be an informer or undercover police officer [i.e. to reverse the decision to sell drugs].

1938		D. Maurer ‘Lang. of the Und. Narcotic Addict’ Pt 2 in Lang. Und. (1981) 99/1: To back up […] 2. (intr.) To refuse to make a connection or to take drugs because of suspicion that the peddler or the addict is a stool-pigeon.
1989		J. Morton Lowspeak.

4. (drugs) to distend the vein during drug taking, thus making it easier to insert the needle [? SE put one’ s back up; the distension of the vein raises it above the surrounding flesh].

1938		D. Maurer ‘Lang. of the Und. Narcotic Addict’ Pt 2 in Lang. Und. (1981) 99/1: To back up. 1. (trans.) To distend the vein into which the addict intends to inject narcotics by holding up the circulation with a string or rope and massaging the skin over the vein.
1949		Monteleone Criminal Sl. (rev. edn).
1959		J.E. Schmidt Narcotics Lingo and Lore.
1989		J. Morton Lowspeak.
2001		ONDCP Street Terms 2: Backup — To prepare a vein for injection.

5. (drugs, also back) to pump the hypodermic so that blood comes into the tube, mixing with the drug/water solution before shooting it back into the vein [i.e. to reverse the movement of the plunger].

1968		N. Heard Howard Street 230: He backed the dope, shooting in slowly, then letting the dropper fill only to shoot it back again.
1981		D.E. Miller Bk of Jargon 339: back up: To repeatedly inject and withdraw blood mixed with heroin while the syringe is in the vein, so as to heighten and lengthen the initial rush.